1. Field of Invention
The present invention relates to a method for analyzing a polishing frequency and a number of polishing times. More particularly, the present invention relates to a method for analyzing an effective polishing frequency and an effective number of polishing times for chemical mechanical planarizing a wafer with different polishing pad profiles.
2. Description of Related Art
Chemical mechanical planarization (CMP) is a global planarization technique which employs both of a mechanical polishing by polishing media and a chemical polishing by chemical solution to remove particles on a wafer surface so that subsequent processes such as deposition and etching are successful. As global planarization is a basic requirement for multilevel interconnects and CMP is recognized as a feasible way to globally planarize a wafer, CMP is very commonly used in semiconductor processes.
In the conventional planarization analyzing technique for a wafer processed by CMP, a finite element method is often used for evaluating a pressure field distribution during polishing. A speed distribution can be obtained from relative velocity between any point on the wafer and a polishing pad, derived by a relative rotating speed. There are also experimental efforts to derive a relation between a speed distribution and a removal rate.
In a typical CMP method, the speed distribution is evaluated under a condition of a planet path and an identical rotating speed for the wafer and the polishing pad. In the case of other relative rotating speeds, an averaged speed distribution is often used. As to compensating chemical mechanical wafer polishing with the wafer disposed above the pad, if the planet path is employed and the wafer and the polishing pad have an identical rotating speed, a distribution of a number of polishing times on the wafer surface is uneven due to the polishing pad incompletely covering the wafer, so that a good planarization cannot be obtained.
Unfortunately, evaluation of the relative speed is based on complex principles and has the following difficulties. Evaluation of the speed for compensating chemical mechanical planarization involves complicated integration, and evaluation of the number of polishing times is difficult, especially for a non-circular polishing pad.
Implementation of global planarization detection is also difficult. For an ordinary chemical mechanical wafer polishing, certain measurement positions on which an endpoint detector measures are selected indirectly. For compensating chemical mechanical wafer polishing, although the polishing surface of the wafer faces upward, which helps a direct measurement during polishing, the available number of measurement positions is still limited and the global planarization detection is not easily achieved because global planarization effect is related to an effective polishing frequency or an effective number of polishing times of all points on the wafer.
For the foregoing reasons, a method for analyzing the effective polishing frequency and the effective number of polishing times for chemical mechanical wafer polishing is needed, providing a reference to the distribution of the effective number of polishing times after chemical mechanical wafer polishing for a period of time.